In a digital imager, pixel power is required to perform operations, such as resetting a floating diffusion and translating charge on the floating diffusion to an output pixel voltage. The pixel power, however, may induce unwanted behavior within the digital imager. Specifically, the digital imager may suffer from effects of hot pixels and dark current.
Hot pixels are generally caused by pixels with higher than normal rates of charge leakage. Hot pixels show up as bright points in an image. Dark currents, on the other hand, are charges accumulated by a pixel even though the pixel is not exposed to light. Thus, dark currents may be seen as off-set noise.
Two contributing causes of hot pixels and dark currents are high floating diffusion voltages and fringing field effects in a drain terminal of a reset transistor. A high floating diffusion voltage is maintained by global anti-blooming functions, since the reset signal is high for unselected rows. A fringing field effect of the drain terminal in the reset transistor is caused by a voltage potential being supplied to the power node of a pixel. The voltage potential on the power node lowers the p-n potential barrier, thus causing charge leakage.
Another unwanted result caused by voltage being supplied to the power node is standby leakage current. Specifically, standby leakage current is dominated by the p-n junction leakage current between the N-epitaxial layer and the P-substrate underneath it, as well as the buried P-layer above it. In addition, leakage current is present between the N+ diffusion areas within the pixel array and the P-epitaxial layer. Such leakage current is particularly undesirable in low-power applications, such as portable devices utilizing image sensors.
It would be an advantage to have a system and method for switching the pixel power. It would also be advantageous if the system and method could provide a high impedance mode.